There are currently a number of piezo-electric vibrating rate sensors being produced for different applications. For instance, piezo-electric vibrating rate sensors are described for use as gyroscopic devices in U.S. Pat. No. 4,655,081, the teachings of which are incorporated herein by this reference. Generally, the vibrating portion of the sensor is made from a man-made material whose properties vary with both temperature and time.
Properties of the vibrating structure vary with both temperature and time. These variations result in a difference between the indicated rate and the actual rate. Therefore a difference exists between the indicated rate of the sensor as determined by demodulating the output signal from the secondary mode of vibration, and the actual rate. These variations make such sensors unsuitable for many applications in which a high degree of accuracy is required.
One method of avoiding the above mentioned problem is to monitor the temperature and time and compensate the output of the rate sensor with a scale factor. Unfortunately, the material properties of the sensors are not exactly reproducible from batch to batch and hence differing variations in the rate sensor and thus the scale factor still present themselves.
Conventionally, a stablized scale factor for the rate sensor is determined by arranging a feedback loop which maintains the output of the primary pick-off amplitude constant as temperature and time change. However, this method does not fully stablize the scale factor. It improves the scale factor stability, but there is still, for typical piezo-electric materials, about a .+-.5% variation in the scale factor due to temperature which is not compensated for using the conventional method.
Therefore, there exists a requirement for a method of and apparatus for obtaining the value of those parameters of a piezo-electric rate sensor which vary with both temperature and time so as to enable an accurate scale factor compensation to be achieved and hence ensure the accuracy of the sensor.